1. Field of the Invention
This invention relates to a process for manufacturing a semiconductor, an apparatus for manufacturing a semiconductor, and an amorphous material suitable as a optical semiconductor for optoelectronics.
2. Description of Related Art
Wide bandgap semiconductor compounds, such as AlN, GaN, AlGaN, GaInN, InN, and the like have lately attracted considerable attention as materials applicable to blue LED, blue LD, and visible light emitters. NH.sub.3 or N.sub.2 is used as a V group element source in manufacturing these nitride type III-V (13-15: Group number defined by inorganic chemical nomenclature revised edition (1989) in IUPAC (International Union of Pure and Applied Chemistry)) semiconductors. However, NH.sub.3 and N.sub.2 are more stable and inactive than other V group element sources such as AsH.sub.3 and PH.sub.3, which are used in manufacturing other III-V semiconductors. Therefore, the substrate temperature is adjusted to from 900 to 1,200.degree. C. when forming a film of a nitride type III-V semiconductor compound on a substrate by metal organic chemical vapor deposition (MOCVD).
On the other hand, In (indium) is hardly incorporated into crystal at all, even at such a high temperature as 900 to 1,200.degree. C., at which GaN of good quality grows. For this reason, the substrate temperature is lowered when manufacturing a mixed crystal containing In. However, with this method, film quality is sacrificed and it is difficult to prepare a mixed crystal of good quality containing 10% or more of In. Also, in a method involving changing the substrate temperature, it is difficult in practice to produce multilayer film type and superlattice type components and the like. The reason for this is that, when forming a film at high temperature on a film formed at low temperature, there is the risk of element diffusion and the like occuring in the film formed at low temperature.
As a method for lowering the growth temperature, disclosed are methods in which a plasma is formed of N.sub.2 or NH.sub.3 as a V group element source by radio frequency discharge, microwave discharge or electron-cyclotron resonance and a metal organic compound containing a III group element is introduced into the remote plasma to form a film (J. M. Van Hore et al, J. Cryst. Growth 150 (1995) 908, A. Yoshida, New Functionality materials, Vol. C. 183-188 (1993) and S. Zembutsu et al, App. Phys. Lett. 48, 870, 1986). As an apparatus for practicing this method, a semiconductor manufacturing apparatus is conventionally known which comprises a plasma generating means communicating with a reactor, a first supply means for supplying a V group element such as N.sub.2 gas to the plasma generating means from the side opposite to the reactor, and a second supply means for supplying a metal organic compound containing a III group element to the reactor side of the plasma generating means.
When a mixed crystal is produced using this semiconductor manufacturing apparatus, mixed gas containing two or more metal organic compounds, for example, trimethylgallium and trimethylindium is supplied by the second supply means. However, when the mixed gas containing these metal organic compounds is introduced into plasma, either of these tends to be selectively decomposed since these metal organic compounds have different bond energies. Therefore, it is difficult to control the composition of the film to be produced even if the ratio of these compounds is controlled, and the resultant film contains carbon impurities derived from the metal organic compound that is more resistant to decomposition.
In addition, when laminating a film of different elements using the above apparatus for manufacturing a semiconductor, raw gas needs to be replaced. It takes from a few minutes to less than 20 minutes for the concentration of an active material to reach the desired value from the time at which raw gas is replaced. The film cannot be formed during that time.
On the other hand, with respect to an amorphous optical semiconductor, an amorphous chalcogenide compound such as selenium, tellurium, or the like is conventionally used as a photoelectric transfer material in camera tubes, receiving optics, photoreceptors and the like as described in "Fundamentals of amorphous semiconductors" published by Ohm Co., Ltd. Recently, hydrogenated amorphous silicon has been used in solar batteries, image sensors, thin film transistors, photoreceptors and the like.
The amorphous chalcogenide compound is, however, thermally unstable and tends to crystallize, so service conditions are limited and hence the valence electron is not well-controlled.
On the other hand, the valence electron of the hydrogenated amorphous silicon can be well-controlled so that the pn junction and the field effect at a boundary layer can be realized. Also, the hydrogenated amorphous silicon has enough thermal resistance to withstand temperatures of up to about 250.degree. C. However, the photoconductivity of the hydrogenated amorphous silicon is weakened by intense light (Staebler and Wronski effect: Handbook of applied physics and the like). Therefore, the efficiency of the hydrogenated amorphous silicon may be reduced in use due to the deterioration caused by light during use in solar batteries and the like. Also, semiconductors composed of these elements, including crystals, are of an indirect transition type and hence there are limitations to their application. For example, they cannot be used in light emitters. Amorphous materials of the III-V compound semiconductors have been investigated as materials able to solve the problems of these amorphous materials.
The amorphous materials of III-V compound semiconductors are formed into a film either by processing a III-V crystal by vapor deposition or sputtering or by reacting an atomic III metal with a molecule or active molecule containing a V group element. Also, a metal organic compound containing a III metal and a metal organic compound containing a V metal are used to form a III-V compound film on a heated substrate (MOCVD). When a crystal film is formed on a substrate using these methods (at 600-1,000.degree. C.), the substrate temperature is set to be lower than the above temperature in order to prepare an amorphous III-V compound. In this case, however, carbon derived from the organic metal remains in the film, and there is a large number of defect levels in the film, and the like and hence there is no amorphous III-V compound able to function as a photoelectric material (H. Reuter, et al, (Thin Solid Films), Vol. 254, pp.96-102 (1995)).
On the other hand, it is known that the density of defect levels between bands in amorphous silicon is reduced by hydrogenation, which allows the valence electron to be controlled. A III-V amorphous compound semiconductor is conventionally hydrogenated by reactive vapor deposition or reactive sputtering (M. Onuki et al, Journal of Non-crystalline Solids Vol. 114, pp.792-794 (1989); U. Coscia et al, Journal of Non-crystalline Solids Vol. 194, pp.103-108 (1996); and the like).
By the introduction of hydrogen, the passivation of dangling bonds is expected, the dangling bond being generated when a III atom and a V atom are combined with each other in a film and become amorphous. However, the resultant film may be sensitive to air so that oxidation tends to occur depending on the bond configuration with hydrogen and the amount of hydrogen introduced.
Also, when hydrogen is introduced for the III-V compound semiconductor, it is known that there is not only the problem with respect to the passivation of the unbonded bands, but also the fear that a dopant for pn control is inactivated in the case of crystal films. In the case of amorphous films, the content of hydrogen in the film and the bonding positions are factors to be considered as problems.